When a communication path is established between a calling and called party, if one of the parties to the call is associated with a key telephone system, it is possible for the key telephone party to place the call in the active but noncommunicating (hold) mode. In this mode the communication path between parties is maintained active but communications cannot take place. The key telephone party can, by operation of the line key, return the call to the bidirectional communication state.
In some situations it is now possible to provide some form of entertainment, usually music, to the party who has been placed on hold. Several problems exist with present music-on-hold circuit arrangements. These problems have to do primarily with the fact that, for economical reasons, the same line circuit must be used to serve all stations whether or not the music-on-hold feature is provided. Another problem arises in that an external source of entertainment signals must be provided and coupled to the line in a manner which allows the switching network to be protected in the event of trouble conditions with the signal source and in a manner which does not cause a longitudinal imbalance in the signal transmission path.
Thus, a need exists in the art for a line circuit arranged for easy adaptation to the provision of entertainment signals over an active noncommunicating telephone line. A further need exists in the art for such a line circuit which is capable of providing externally generated signals, such as music, and for also providing internally generated signals, such as tone-on-hold, while at the same time being arrangeable to provide neither of these signals.